Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
1036 PM EDT Thursday Jul 30 2015
a cold front crosses the area this tonight. High pressure returns
Friday into Saturday. A weak cold front crosses the area Saturday
night...before high pressure returns to start next week.
Near term /until 6 am Friday morning/...
warm temperatures aloft...weak middle level lapse rates and a lack of any
appreciable forcing has resulted in only isolated to widely
scattered showers/thunderstorms late this evening. This despite an
anomalously moist and unstable air mass over the local area.
Dewpoints still hovering in the 70s...but relief is in site as
dewpoints have dropped into the 50s and 60s in western Virginia. The
dewpoint boundary/weak cold front will continue to approach the
region this evening...crossing the region through the overnight
hours. While shear is nearly non-existent...will maintain 30-40
probability of precipitation across the local area as the cold front/dew point boundary
navigates the warm...humid and unstable air mass. Sky begins to
clear west to east Post frontal...with a mostly clear sky expected
inland late tonight. Front locates along the coast late
tonight...with 20-30 probability of precipitation remaining across the coastal plain. Low
temperatures will range from the middle 60s northwest to the low to middle 70s along
Short term /6 am Friday morning through Sunday/...
Friday is looking dry for all but the far southeast parts of the forecast area as
high pressure arrives behind the departing front. The humidity
will be lower as well. Have 20% probability of precipitation far southeast...closest to the
stalling frontal boundary. Highs in the upper 80s to low
90s...except middle 80s coast.
Friday night and Saturday...
high pressure will remain over the area for dry weather and a mostly
clear to partly cloudy sky. Humidity levels start to increase
a little on Sat. Lows Friday night in the 60s to lower 70s. Highs on
Sat in the low 90s...except middle 80s coast.
Saturday night and Sunday...
a weak cold front is now shown to move across the area Saturday
night. What remains to be seen is whether or not it can muster up
enough moisture to produce a few showers as is passes. For
now...have left the forecast dry as models not all the convinced
that it will. Mainly clear to partly cloudy with lows in the middle 60s to
low 70s. Remaining dry most places on Sunday as high pressure
rebounds. There's a small (20%) chance of isolated showers across the
far southeast with a weak coastal boundary still in place. Highs Sunday
in the upper 80s to low 90s...except middle 80s coast.
Long term /Sunday night through Thursday/...
pattern begins with a middle/upper level ridge centered over the SW
Continental U.S. And an upper level low near Hudson Bay Canada. GFS/European model (ecmwf)
are in good agreement with this overall setup into Tuesday...but then
diverge quite a bit Wed/Thu...the operational GFS being much
stronger/pushing farther south with the upper trough than the
European model (ecmwf) (or the GFS ensemble means). Typical summertime temperatures
can be expected Monday with highs mainly ranging from the middle
80s/around 90 f along the coast to the lower 90s inland. Generally
looks dry although will maintain a 20-30% pop in NE NC due to
potential seabreeze. For Tue-Wed...fairly confident in hot weather for
Tuesday...as both models have rising 800 mb temperatures and a west/SW flow in low
levels. Will maintain a 20% pop due mainly to a weak cap with 500 mb
heights only 585-588 dm. Highs mainly in the middle 90s inland and
90-95 f at the coast. Less confidence for Wed/Thu...GFS would
suggest much cooler conds...while the European model (ecmwf) would support highs in
the middle 90s to perhaps 100 f. While the European model (ecmwf)/wpc pattern will
generally be favored (cold front to our north until Thursday aftn)...think
the drier/hotter weather will ultimately not be as hot as predicted due
to antecedent wet/humid conditions of the past month. For now will
show a slight cooling with highs Wednesday 90-95 f and Thursday upper 80s to
lower 90s. Dew points are forecast by models to mix out into the low
60s west of I-95 through the period...generally stayed a little higher
than this...middle 60s west to lower 70s east. For now this yields heat
indices staying below 105 f next week. As for probability of precipitation...will only
have ~20% probability of precipitation Wednesday...rising to 30% most areas Thursday west/ the approach
of the cold front from the north.
Aviation /02z Friday through Tuesday/...
as of 00z...widely scattered showers/thunderstorms have diminished but have not
ended and will threaten some of the taf sites during the remainder
of the evening. A cold front near Charlottesville and southeast of Roanoke
will push southeast and forecast to pass through ric by 06z and ecg
around 12z. MOS has MVFR visibilities at sby throughout the evening and
overnight. Have MVFR at sby from 04 to 08z. North/northwest winds develop
behind the cold front. A mostly clear sky will prevail on Friday.
Outlook...dry weather prevails Saturday. A slight chance of thunderstorms is in
the forecast Sunday through Tuesday...especially southern portions.
latest observation reflect southeast flow across the waters this afternoon...shifting
to the S/SW and to average ~15 knots this evening. Winds turn to the north-northwest
after midnight through Friday morning. Not anticipating a strong
cool surge on Friday but will not take much to get ~15 knots winds Friday
morning with surface water temperatures at or above 80 f over the Bay/rivers
(staying just below Small Craft Advisory thresholds for now but would not be
surprised to see a few hours with gusts to 20 knots Friday morning).
Expect waves in the Bay 2-3 feet this evening drop off to 1-2 feet after
midnight then increase back to 2-3 feet Friday morning before diminishing
Friday afternoon. Seas average 3-4 feet north and 2-3 feet S. Winds shift to the north-northeast
midday Friday...becoming east by late in the day/Friday evening. Winds then
turn back to the south-southwest for Friday night and Sat...as the front weakens
just to our south-southeast along the Carolina coast. A weak front now prognosticated
to cross the area Sat night/early sun...before turning back to the
south late sun into next week as surface high pressure becomes
anchored over the western Atlantic. Waves Sat into next week average
1-2 feet in the Bay with seas 2-4 feet.
southerly channeling has resulted in some nuisance-type tidal anomalies
on the order of a half to three quarters of a foot above normal at
Bishops Head/Cambridge. Water levels look to approach, but still fall
short of minor flood thresholds with astronomically higher tide
cycle late tonight. Falling Water levels thereafter as flow turns
northerly into Friday morning. Anomalies may rise again late Sat/Sat
night in S flow up the Bay ahead of another weak cold front.